The present invention relates to a breakable composite drill screw which can be snapped in two in such a manner that a drill bit or chip and a superfluous threaded stem portion adjoined thereto are broken off to be removed from the drill screw after it has been turned tight.
A so-called drill screw which has a drill bit formed at a tip end of a screw having a driving head is advantageous in that the forming of holes in workpieces to be fastened, tapping of the holes and fastening of the workpieces with the screw can be carried out in one continuous operation. Therefore, the range of its use is rapidly increasing. As new and various uses are found for such drill screw, there arises a certain problem in some cases. For example, the drill bit and a superfluous portion of threaded stem adjoining said drill bit protrude from a back surface of a structure after it has been fastened by the drill screw, thereby spoiling the beauty of the structure or building and further being likely to inflict an injury on a person. This has led to a serious demand for an improved drill screw which after fastening thereof permits its protruding super fluous portions to be broken and removed from its main portion.
In this regard, it is a common practice to form a groove at a point where a screw is to be broken to remove such protruding superfluous portions. However, such a common practice cannot be applied to aforementioned drill screws because they have previously been so hardened as to have sufficient hardness for the machining or cutting of their drill bits and also for the tapping of their threaded stems adjoining the drill bits. The thus hardened surface makes it difficult to break the screw at such grooved portion thereof.